Displacement sensors are used in a variety of applications to measure the displacement of an object. Displacement sensors may measure a linear displacement of objects, or may measure a rotary displacement of objects. One type of displacement sensors operate on the principal of a transformer, and are referred to as Rotary or Linear Variable Differential Transformer sensors, hereinafter referenced as RVDT sensors, LVDT sensors, or collectively referenced as R/LVDT sensors.
An LVDT sensor, in one exemplary configuration, includes a primary winding that is wound around a hollow form. The LVDT sensor also includes secondary windings that are wound around the hollow form overlapping or on either side of the primary winding. As a power source applies an AC voltage to the primary winding, the magnetic flux produced by the primary winding is coupled to the secondary windings. This induces an AC voltage in each of the secondary windings. The LVDT sensor also includes a core that is able to move along a linear axis within the hollow form. The core of the LVDT sensor is attached in some manner to the object whose displacement is being measured. As the core moves within the hollow form due to movement of the object, the primary winding will be more strongly coupled to one of the secondary windings. The differential voltage across the secondary windings therefore indicates a displacement of the core in the LVDT sensor.
A four-wire LVDT sensor includes two input wires for the primary winding, and two output wires for the secondary windings. The output wires allow for a measurement of the differential voltage across the two secondary windings. One problem with four-wire R/LVDT sensors is that it is difficult to determine when these sensors have failed. This is because, when the core of the R/LVDT sensor is in a neutral or intermediate position between the two secondary windings, the primary winding will be equally coupled to each of the secondary windings so that the secondary windings will have the same voltage amplitude and an opposite phase. Thus, when the differential voltage across the secondary windings is demodulated into a DC voltage, the DC voltage will be zero volts. Because zero volts is a valid output, it can be difficult to directly detect a failure in a four-wire R/LVDT sensor.